Preheat a grill to medium. Meanwhile, make the salsa: Toss the tomatoes, tomatillos, red onion and the remaining 3 teaspoons lime juice in a bowl; season with salt. Cover and refrigerate until ready to serve.

Brush the grill grates with olive oil. Grill the chicken, turning once, until marked and just cooked through, 4 to 6 minutes per side. Top with the salsa and serve with the lime wedges.